When a data pulse is applied to a laser diode such as in an optical communication system, the laser diode switches between an OFF condition where very little current flows through the diode and there is no substantial lightwave or optical output signal and an ON condition where a large amount of current flows through the laser diode to cause it to lase and produce an optical output signal. As the laser changes between the OFF condition and ON condition, a shift occurs in wavelength. This shift is commonly called "chirp." This wavelength shift or chirp causes the optical pulse to disperse, or smear in time, as the pulse travels through the optical fiber. This phenomenon makes it difficult to distinguish between a logic 0 level and a logic 1 level in the signal at the optical receiver. This difficulty may be termed "bit-error rate degradation."
In high data rate optical communication systems, laser chirp characteristics impose a significant limitation on the maximum obtainable transmission distance. In order to minimize laser chirp, the amplitude of the non-return-to-zero (NRZ) data modulation applied to the laser must be controlled to avoid driving the NRZ data logic zero level into the laser threshold region. This restriction prevents the use of known laser threshold sensing modulation control techniques that are used at lower data rates.
Existing approaches for preventing bit-error rate degradation due to chirp in high-speed optical communication systems include (1) techniques that use laser threshold sensing (2) techniques that add a low frequency control tone to the NRZ data logic 1, and (3) approaches that monitor and measure the laser gain at the laser diode back facet. These solutions have significant limitations as the following discussions explain.
Threshold sensing techniques are unsatisfactory because they require the laser diode to operate in the threshold region. Laser operation in the threshold region, by its very nature, produces excessive chirp. So, this approach is an unsatisfactory way to prevent bit-error rate degradation.
Adding a low-frequency tone to the logic 1 of the NRZ data stream in high speed systems does not work well, because it is difficult to add a low-frequency tone to only one side of the NRZ data stream. The technique of adding a control tone to the logic 1 is also expensive, at least in part because the technique requires the use of radiofrequency devices for logic 1 signal level manipulations. Reliability is also less than desirable signal level in these techniques because the radiofrequency devices that can accomplish this detection are fragile. These techniques also generally require a good phase match between the primary high speed data path and the secondary logic/manipulation path over the communication system temperature and voltage operating ranges. This phase match requirement even further adds to the difficulty of accomplishing this technique.
Techniques that measure the laser gain by monitoring the high speed data with a back facet monitor also do not work well for numerous reasons. One reason is that back facet detectors generally have limited bandwidth. Also, the amount of high speed signal data at the laser back facet that is available for detection is small. Back facet control systems that monitor these high speed signals, therefore, must recover a small signal and attempt to monitor signals that may have frequencies up to 1.2 GHz. These techniques, therefore, require a significant amount of broadband gain to recover the NRZ data from the laser diode back facet signal. In addition, these techniques require coupling high speed electronic circuits with the laser diode back facet. High speed electronic circuits that can couple with the laser back facet, however, are generally expensive and unreliable. Due to its broadband nature the inherent detector limitations, and the type of signal that this approach detects, this approach does not produce a sufficiently high signal-to-noise ratio to ensure stable modulation level control.
Gain stability over normal operating temperature and voltage ranges is yet another difficulty that systems employing these techniques experience. That is, as temperature or voltage changes, drift in the NRZ signal level occurs due to gain changes in the associated broadband amplifiers. This gain instability adversely affects detector accuracies. As the drift increases, so do the adverse effects, that lead to detector accuracies. These inaccuracies make monitoring high speed data to measure laser gain an unacceptable technique.
An important aspect of controlling the operation of a laser circuit associates with a laser modulation controller is modulation of the electrical input into the laser. By modulating the electrical input it is possible to ensure a constant average power level into the laser as well as to accommodate or changes in non-return to zero data that reaches the communications laser circuit.
There is a need, therefore, for a laser modulation controller that modulates the electrical input into the laser circuit.